This invention relates to a novel class of curing catalysts having a heat-latency, i.e. which are normally inactive but are capable of initiating a curing reaction only at an elevated temperature. The invention also relates to heat-curable resin compositions containing these catalyst which are useful for the preparation of coatings, adhesives, printing inks and other compositions.
In a heat-curable coating system utilizing a proton-donating aromatic sulfonate catalyst, it is imperative to block the sulfonate catalyst with a volatile amine for preventing a premature curing reaction from occuring during storage.
The amine blocked sulfonate catalyst releases a proton according to the following scheme: EQU RSO.sub.3.sup.-.H.sup.+ N(R).sub.3 .revreaction.RSO.sub.3.sup.- +H.sup.+ N(R).sub.3 .revreaction. EQU RSO.sub.3.sup.- +H.sup.+ +N(R).sub.3
Accordingly, when it is desired for the sulfonate to exhibit its function at a relatively low temperature, the amine-to-sulfonic acid ratio must be decreased. This increases the amount of free sulfonic acid with the result being decrease in the storage stability of the entire system. Conversely, if the amine-to-sulfonic acid ratio is increased so as to enhance the storage stability, the amount of amine sulfonate salt increases accordingly. This requires the use of higher curing temperatures than desirable so that the sulfonate catalyst functions to its fullest extent. Thus, the curability of a curable system utilizing the amine blocked sulfonate catalyst is generally not compatible with the storage stability thereof.
Accordingly, a strong need exists for a proton-donating catalyst which obviate the above defects.